Friday, February 20, 2009

Not too much to say about this fragment. It's a limey sandstone/siltstone with several little faults running through it. I picked it up on last spring's trip to the California desert- I think near Darwin (east side of Owens Lake), but I'm not really sure. The offset on the pair visible here look like reverse faults. However, if you look carefully, the width of the fault is similar to the total offset.Now technically, a joint is a fracture that involves little or no offset parallel to the fracture surface. Place your palms together, then pull them apart a half an inch or so: that represents a joint. Put them back together, and slide one hand some direction without separating the surfaces: that represents a fault. In this sample, the separations both parallel and perpendicular to the fracture surface look to be about the same, so what should I call it?Looking at the side of the fragment (this is the side that was closest to the quarter in the first picture), you can see there's actually a larger fault running through the middle of the rock. My suspicion is that the two cute little microfaults visible in the first picture are more acurately described as brecciation (breaking, shattering, crushing) associated with this larger fracture. Brecciation that, in this case, appears to have a reverse offset. I'll still call them faults, because I'm rather fond of tiny little samples that capture what are normally very large features, but in this case, I'm quite unsure just how accurate I am in applying that name to these features.

This leads me to an important theme in geology: names and terms can be very useful in communication, but they can also get in the way of understanding if naming something becomes an inaccurate stand-in for actually describing it. What I want to do is understand how the rock formed, how it came to have the features it does. Noticing the features is necessary to do that, but naming them isn't necessarily necessary.

And regarding all the equivocation above, all I can do is apologize and repeat an old joke: "The world needs more one-armed geologists, so we don't have to hear, 'on the other hand,' every few sentences."